1. Field of the Invention
The present invention relates to novel materials that are superconducting at high temperatures, hereinafter designated as "HT superconductors", and to the production of such novel HT superconductors having stabilized, and even enhanced superconducting properties, by low temperature fluorination.
2. Description of the Prior Art
It is known to this art that, in theory, when superconductors are cooled to very low temperatures, they offer no resistance to the flow of an electric current, and, when subjected to an external magnetic field, diamagnetic properties result.
Until recently, the state of superconductivity was observed only in certain materials below a threshold temperature, designated the "critical temperature", such threshold typically approximating absolute zero. This type of limitation obviously presents a serious drawback in the large scale development of any potential applications resulting from superconductivity.
Very recently, however, research has given rise to the development of new materials having superconducting properties at higher temperatures.
More specifically, the recent discovery of oxygen containing materials based on copper, rare earths (lanthanum, yttrium, erbium, etc.) or bismuth or thallium, and alkaline earth metals (barium, strontium, calcium, etc.) having high critical temperatures (Tc) paved an unexpected research path in the field of HT superconductivity.
In the year 1987, for example, a veritable avalanche of results relative to the synthesis and the chemical and physical properties of a great number of novel superconducting materials was published.
Among the most frequently studied systems, Y--Ba--Cu--O is particularly representative, in which the phases derived from YBa.sub.2 Cu.sub.3 O.sub.7 have critical temperatures on the order of 92.degree. K.
Certain publications have even featured superconductors that were operational at temperatures close to or even higher than ambient temperature. In the latter case, however, no satisfactory test results were described.
Such materials present possibilities in widely varying fields, such as high-speed microelectronics, due to the elimination of the resistivity of connections, the manufacture of superconducting storage coils, in the use of superconductors having high critical temperatures for transportation purposes which entail electromagnetic levitation, and, as a general rule, in all applications which at the present time require the use of superconducting magnets.
But the improvement in the performance of HT superconducting materials and, in particular, increasing their critical temperature, mandates the stabilization of phases possessing an amount of perfectly defined anionic vacancies, or, in other words, perfect stability relative to the ambient temperature.
It too is well known to this art, on the other hand, that surface defects interfere with the intrinsic properties of such materials.